JP2545189B2 - Harmonic mode lock laser device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse generator, and more particularly to a harmonic mode lock ring laser.

[0002]

2. Description of the Related Art US Pat. No. 4,700,339 (1) by Gordon et al.
(October 13, 987) and Mollenauer et al., US Pat. No. 4881790 (November 21, 1989).
Is an example of a document showing the advantages of using solitons in long-distance optical fiber transmission of information. A soliton is an optical pulse that has a specific shape, pulse width, and energy. At this time, each pulse can propagate over a long distance in a single-mode optical fiber while maintaining the pulse shape and pulse width. It is possible. In propagation in a single-mode glass optical fiber, it is ideal that such a pulse shape has an intensity envelope that is the square of the hyperbolic secant. Such pulses compensate for the effects of chromatic dispersion in the fiber due to some non-linear effects in the single mode fiber. A pulse with the smallest time-bandwidth product for a given spectrum, which is the characteristic of an ideal soliton pulse, is
Another advantage of soliton transmission is the practicality of using an erbium amplifier rather than a repeater to amplify the transmission signal. With soliton transmission, the transmission line capacity can be further improved. ,
Wavelength division multiplexing and polarization division multiplexing can be used.

"Mode-Locked" by JDKafka, T. Baer
Erbium-Doped Fiber Laser with Soliton Pulse Shapin
g, "Optics Letters, Vol.14, No.22, November 15, 198
9, pp.1269-1271 and A. Takada, H. Miyazawa, "P
icosecond Pulse Generationfrom Actively Mode-Locke
d Erbium-Doped Fiber Laser, "Electronics Letters, V
ol.26, No.3, February 1, 1990, pp.216-217 is an example of a paper using a harmonic mode-locked fiber ring laser for generating a short pulse. The ring laser shown in the above paper uses a pump light source for pumping an erbium fiber amplifier placed in a closed-loop resonator transmission line. The electro-optic modulator in the transmission line is driven at an appropriate frequency, and the laser light is made into an optical pulse having a repetition rate that is a harmonic of the frequency interval between adjacent resonance modes of the closed-loop resonator transmission line. . Optical pulses circulating in the closed loop of a ring laser are coupled out of the closed loop to form a pulse that can be used for soliton transmission. It is known that the optical pulse generated from an active mode-locked laser tends to have a Gaussian shape, and this pulse shape has a desired hyperbolic secant squared pulse shape that can be used in a soliton transmission system. They are equal enough. Unfortunately, we have found that pulse trains generated from harmonic mode-locked ring lasers tend to have amplitude fluctuations. Amplitude fluctuations and other instabilities of these pulses give rise to inherent and unwanted interactions between modes in the closed loop of the ring laser.

[0004]

Therefore, there is still a need for a practical optical pulsed light source for use in fiber optic transmission and especially as a practical soliton light source.

[0005]

According to the present invention, there is provided a means for forming a closed loop optical transmission line, an active laser medium for emitting coherent light along the optical transmission line, and an optical transmission line arranged in the transmission line. It relates to an improvement of a harmonic mode-locking laser of the type having a means for propagating a transmission line as a train of light pulses having a period sufficiently equal to an integer fraction of the time that the light pulses pass. An improvement in the present invention is to place a Fabry-Perot optical resonator in a closed loop transmission line so that the free spectral range of the optical resonator is sufficiently equal to the pulse repetition rate of the optical pulse in the ring laser. Under this condition,
The Fabry-Perot resonator acts to equalize the energy, pulse shape, and pulse width of the individual pulses.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the embodiment of the harmonic mode lock ring laser according to the present invention, as shown in the drawing, a part of the closed loop optical transmission line is constituted by a single mode optical fiber 12. The optical coupler 13 guides the optical energy from the pump laser 14 to the erbium fiber amplifier 15. The erbium fiber amplifier 15 is, for example, SB
Poole et al., "Fabrication and Characterization o
f Low-Loss OpticalFibers Containing Rare-Earth Ion
s, "Journal of Lightwave Technology, Vol.LT-4, No.
7, July 1986, pp.870-875, which is an active laser medium of the type shown in FIG. 7, which is composed of an optical fiber portion doped with erbium and along which the energy from the pump laser 14 is properly Optical amplification occurs when pumped. Various other types of active laser media can be used for optical amplification in harmonic mode-locked ring lasers. The optical energy propagates along the closed loop defined by the optical transmission line, and the polarization controller 17 is used to correct the polarization fluctuation in the ring. Polarization controllers, as known in the prior art,
It is constituted by an optical fiber loop of appropriate length for conversion to an appropriate polarization state; if the polarization state of the light circulating around the ring does not matter, the controller 17 is not required.

The modulator 18 modulates the light intensity of coherent light to obtain an optical pulse train. Normally, the Mach-Zehnder lithium niobate modulator is driven by the electric oscillator 19 at a frequency of the gigahertz band to form an optical pulse train having a corresponding repetition rate. The modulation frequency is set so that the pulse repetition rate has a period sufficiently equal to an integer fraction of the transit time of the optical path of the optical pulse, that is, T = t / N is satisfied, where , T is the period of the optical pulse, t is the transit time of the optical pulse in the closed loop optical path, N
Is an integer. As shown in FIG. 2, the closed loop optical path has a plurality of resonant frequency modes, R1, R2, R3, ... Rn. According to the above equation, the pulse repetition rate f2 is approximately given by f2 = Nf1, where f1 is given in FIG.
It is a frequency interval between adjacent resonance modes as shown in FIG.

The adjustable delay line 21 can be adjusted so that the transit time in the ring is equal to an integer multiple of the modulation period, so that the harmonics of the drive frequency match the longitudinal mode of the ring. To do. The wavelength of the laser is controlled by the tuning element 22, and as in the prior art, it may be composed of an angle adjustment bandpass interference filter for coarse adjustment of the light wavelength and a solid etalon for fine adjustment.

In the present invention, a part of the ring or the closed loop is composed of the Fabry-Perot resonator 24 composed of the semi-transmissive mirrors 25 and 26, and has the function of making the amplitude of the output light pulse uniform as described below. is there. A portion of the pulse train circulating in the ring is extracted from the ring by output coupler 27 and transmitted as an output pulse as shown. In the present invention, these light pulses can be used for soliton transmission into single mode optical fibers, with the known benefits of soliton transmission of information. Isolator 28
Are placed in the ring so that light propagates counterclockwise in the ring as described above.

As mentioned above, a harmonic mode-locked ring laser is capable of producing a pulse having a Gaussian shape, which is sufficiently equal to a hyperbolic secant squared pulse-shaped soliton pulse that can be used in a soliton transmission system. . That is, the nonlinear effect of the optical fiber and the wavelength dispersion effect become equal, and it becomes possible to transmit over a long distance without deteriorating the pulse shape. Unfortunately, it was discovered that the pulse train generated from the conventional harmonic mode-locked ring laser includes an unnecessary oscillation mode, and the intensity or amplitude of the output light pulse tends to be nonuniform. The influence of such fluctuations is great, and at least it cannot be used as a soliton pulse light source.

In the present invention, these problems are solved by using the Fabry-Perot resonator 24, which has a free spectral range well equal to the pulse repetition rate of the optical pulses in the ring laser. The Fabry-Perot resonator has a plurality of equidistant resonance points and a free spectral range (FSR) determined by the frequency intervals of two adjacent resonance modes. When the frequency mode of the Fabry-Perot resonator is shown in FIG. 2; FSR equals f1.
The FSR is also given by FSR = c / 2d, where c is the speed of light in the resonator and d is the optical path between the opposing mirrors 25 and 26.

In the present invention, the resonator 24 has an FSR sufficiently equal to the pulse repetition rate f2 of the ring laser, that is, FSR = f2. In this condition, the Fabry-Perot resonator equalizes the energy transmitted by the pulse train and thus the pulse amplitude of the generated pulse train. Of the ring laser modes, the one that most closely matches the resonance peak of the resonator 24 emits laser light, and the other competing modes are suppressed. Formulas (3) and (4)
As shown in, the orbit time of the light in the resonator 24 corresponds to the desired pulse period.

In order to completely suppress the unwanted modes of the ring, the mode line width of the Fabry-Perot resonator needs to be about the ring mode frequency interval or less. This can be done by making the reflectivity of the Fabry-Perot cavity mirror sufficiently high. Other optical cavities may be used instead of the Fabry-Perot cavities 24 if they satisfy the above-mentioned essential performance of the Fabry-Perot cavities. However, the Fabry-Perot resonator has the most practical configuration and is suitable for implementing the present invention as described above.

A ring laser experimental system according to the present invention was constructed and operated at a wavelength of 1.555 microns. A pump light power of 100 mW was applied at a wavelength of 1.48 microns. The total length of the closed loop is 20 m. The optical coupler 13 is a commercially available product and is manufactured by JDS Fitel Company of Napean, O
Available from ntario, Canada as model number WD1415M1-A1. The erbium fiber amplifier 15 has a standard amplifier configuration and a total length of 15 m. The polarization controller 17 is known as a rotary paddle type controller having three paddles, each having two 1 inch diameter fiber loops. The tuning element is a tuning etalon consisting of a 0.5 mm thick parallel quartz plate with a 73% reflectance coating at a wavelength of 1.555 microns and a center wavelength of 1560 nanometers at normal incidence with a passband of 2 nanometers. The tilt tunable filter of the Virgo Optic
s, Port Ritchie, Florida, and Omega Optical, Bratt
Commercially available from leboro, Vermont. The 2.5 GHz Fabry-Perot resonator 24 uses a 98% reflectivity mirror and has a radius of curvature of 75 mm and a spacing of 60 mm. Isolator 28 and lithium niobate modulator 18 are standard commercial products. The pulse width is 35 to 1 by adjusting the modulation factor.
Easy control up to 00 picoseconds or more. The pulse was essentially transform-limited, approximately Gaussian-shaped, with a time-band product of 0.44. It is considered possible to generate even shorter pulses by appropriately increasing the bandwidth of the tuning element.

[0015]

As described above, the present invention relates to an improvement of a harmonic mode-locked ring laser, in which a Fabry-Perot optical resonator is arranged in a closed loop transmission line, and the free spectral range of the optical resonator is set within the ring laser. The energy, pulse shape of the individual emitted pulses, by sufficiently equaling the pulse repetition rate of the optical pulses of
It is possible to provide a soliton pulse light source that can make the pulse width uniform.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of a harmonic mode lock ring laser according to the present invention.

2 is a diagram showing modes obtained by the ring of FIG. 1. FIG.

[Explanation of symbols]

 12 Single Mode Optical Fiber 13 Optical Coupler 14 Pump Laser 15 Erbium Fiber Amplifier 17 Polarization Controller 18 Modulator 19 Electric Oscillator 21 Delay Line 22 Tuning Element 24 Fabry-Perot Resonator 25 Mirror 26 Mirror 27 Output Coupler 28 Isolator

 ─────────────────────────────────────────────────── ——————————————————————————————————————————— Inventor Lin Frederick Molnour United States 07722 New Jersey Mannmouth County, Colts Neck, Carriage Hill Drive 11

Claims (6)

(57) [Claims] 1. A closed loop optical transmission line (12) and an active laser medium (15) arranged in the optical transmission line.
A means (14) for causing the active laser medium to emit coherent light of a predetermined wavelength; and the emitted light, which is arranged in the optical transmission line, is sufficiently uniform near the optical frequency along the optical transmission line. Means (18, 19) for propagating as a pulse train having a high repetition rate (f ₂ ), an output coupler (27) for removing a part of the pulse from the optical transmission line, and the like arranged in the optical transmission line An optical resonator (24) having an interval resonance point, and a time (t) for an optical pulse to propagate through the closed loop length is substantially equal to an integer multiple of the period (T) of the pulse repetition rate, Frequency interval (f ₁ ) between resonance points (R ₁ , R ₂ )
A harmonic mode-locked laser device characterized in that an integer multiple of is substantially equal to the pulse repetition rate. 2. The apparatus of claim 1, wherein the pulse train is sufficiently equal to the transform limit pulse used as a soliton light source. 3. Device according to claim 1, characterized in that the optical resonator is a Fabry-Perot resonator. 4. Device according to claim 1, characterized in that the active laser medium is a rare-earth-doped single-mode optical fiber. 5. A means (14) for causing an active light medium to emit light.
The device of claim 1, wherein is a means for providing optical pump energy to the closed loop optical transmission line. 6. The ratio between the width of the resonance point and the distance between the resonance points is approximately equal to the ratio of the pulse repetition rate period to the time for which light passes through the closed loop.
An apparatus according to claim 1.